Variation in virulence and its relation to hyphal growth and enzyme activity of Raffaeleaquercivora, a Japanese oak wilt pathogen.

The pathogenic fungus Raffaelea quercivora kills members of the Fagaceae throughout Japan. Although the amount of host damage caused by different strains of the pathogen varies, the degree of this variation and the underlying factors have not been thoroughly examined. We used 40 R. quercivora isolates collected across Japan from different oak and other Fagaceae species and evaluated their virulence in the stem of Konara oak (Quercus serrata), compared their hyphal growth in living and gamma‐irradiated Japanese oak (Quercus crispula) twigs and evaluated their activity of carbohydrate‐degrading enzymes. Of the 40 isolates examined, half were considered avirulent as the width of sapwood discoloration induced by inoculation did not differ significantly from the control. The extension of hyphal growth in living twigs correlated positively with the width of discoloured sapwood induced by inoculation in living twigs and with the extension of hyphal growth in gamma‐irradiated twigs. Activity of starch‐ and pectin‐degrading enzymes was positively correlated with the width of discoloured sapwood induced by inoculation. These results suggest that virulence variation among R. quercivora strains is at least partially related to their ability to grow in sapwood and feed on easily accessible carbohydrates. [ABSTRACT FROM AUTHOR]